King under the Mountain

King under the Mountain was the title taken by Thráin I, founder of the Dwarf-kingdom of Lonely Mountain, and maintained by those Kings of Durin's folk who dwelt there. The line was broken twice, once by Thorin I (he and four generations of his descendants ruled from the Grey Mountains, not Lonely Mountain), and once by the DragonSmaug (who claimed the title for himself). In both cases, the line was restored to a rightful heir.

Inspiration

The term "King under the Mountain" is a philological term for a certain motif in various mythologies and folklore around the world. It refers to a kingly figure who remains hidden in a mountain or cave until a messianic return at the appointed time.